Transactions and Concurrency Control

1. Transactions and Concurrency Control

2. What is a Transaction? • Any action that reads from and/or writes to a database may consist of • Simple SELECT statement to generate a list of table contents • A series of related UPDATE statements to change the values of attributes in various tables • A series of INSERT statements to add rows to one or more tables • A combination of SELECT, UPDATE, and INSERT statements

3. What is a Transaction? • A logical unit of work that must be either entirely completed or aborted • Successful transaction changes the database from one consistent state to another • One in which all data integrity constraints are satisfied • Most real-world database transactions are formed by two or more database requests • The equivalent of a single SQL statement in an application program or transaction

4. The Relational Schema for a Database

5. Evaluating Transaction Results • Not all transactions update the database • SQL code represents a transaction because database was accessed • Improper or incomplete transactions can have a devastating effect on database integrity • Some DBMSs provide means by which user can define enforceable constraints based on business rules • Other integrity rules are enforced automatically by the DBMS when table structures are properly defined, thereby letting the DBMS validate some transactions

6. Transaction Processing • For example, a transaction may involve • The creation of a new invoice • Insertion of an row in the LINE table • Decreasing the quantity on hand by 1 • Updating the customer balance • Creating a new account transaction row • If the system fails between the first and last step, the database will no longer be in a consistent state Figure 9.2

7. Transaction Properties • Atomicity • Requires that all operations (SQL requests) of a transaction be completed; if not, then the transaction is aborted • A transaction is treated as a single, indivisible, logical unit of work • Durability • Indicates permanence of database’s consistent state • When a transaction is complete, the database reaches a consistent state. That state can not be lost even if the system fails

8. Transaction Properties • Serializability • Ensures that the concurrent execution of several transactions yields consistent results • Multiple, concurrent transactions appear as if they executed in serial order (one after another) • Isolation • Data used during execution of a transaction cannot be used by second transaction until first one is completed

9. Transaction Management with SQL • ANSI has defined standards that govern SQL database transactions • Transaction support is provided by two SQL statements: COMMIT and ROLLBACK • ANSI standards require that, when a transaction sequence is initiated by a user or an application program,it must continue through all succeeding SQL statements until one of four events occurs

10. Transaction Management with SQL • A COMMIT statement is reached- all changes are permanently recorded within the database • A ROLLBACK is reached – all changes are aborted and the database is restored to a previous consistent state • The end of the program is successfully reached – equivalent to a COMMIT • The program abnormally terminates and a rollback occurs

11. The Transaction Log • Keeps track of all transactions that update the database. It contains: • A record for the beginning of transaction • For each transaction component (SQL statement) • Type of operation being performed (update, delete, insert) • Names of objects affected by the transaction (the name of the table) • “Before” and “after” values for updated fields • Pointers to previous and next transaction log entries for the same transaction • The ending (COMMIT) of the transaction

12. The Transaction Log • Increases processing overhead but the ability to restore a corrupted database is worth the price • If a system failure occurs, the DBMS will examine the log for all uncommitted or incomplete transactions and it will restore the database to a previous state • The log it itself a database and to maintain its integrity many DBMSs will implement it on several different disks to reduce the risk of system failure

13. A Transaction Log

14. Concurrency Control • The coordination of the simultaneous execution of transactions in a multiprocessing database is known as concurrency control • The objective of concurrency control is to ensure the serializability of transactions in a multiuser database environment

15. Concurrency Control • Important  simultaneous execution of transactions over a shared database can create several data integrity and consistency problems • The three main problems are: • lost updates • uncommitted data • inconsistent retrievals

16. Normal Execution of Two Transactions

17. Lost Updates

18. Uncommitted Data Problem • Uncommitted data occurs when two transactions execute concurrently and the first is rolled back after the second has already accessed the uncommitted data • This violates the isolation property of transactions

19. Correct Execution of Two Transactions

20. An Uncommitted Data Problem

21. Inconsistent Retrieval Problem • Occur when a transaction calculates some aggregate functions over a set of data while transactions are updating the data • Some data may be read after they are changed and some before they are changed yielding inconsistent results

22. Retrieval During Update

23. Transaction Results: Data Entry Correction

24. Inconsistent Retrievals

25. The Scheduler • Special DBMS program: establishes order of operations within which concurrent transactions are executed • Interleaves the execution of database operations to ensure serializability and isolation of transactions • To determine the appropriate order, the scheduler bases its actions on concurrency control algorithms such as locking and time stamping

26. The Scheduler (continued) • Ensures computer’s central processing unit (CPU) is used efficiently • Default would be FIFO without preemption – idle CPU (during I/O) is inefficient use of the resource and result in unacceptable response times in within the multiuser DBMS environment • Facilitates data isolation to ensure that two transactions do not update the same data element at the same time

27. Read/Write Conflict Scenarios: Conflicting Database Operations Matrix • Transactions T1 and T2 are executing concurrently over the same data • When they both access the data and at least one is executing a WRITE, a conflict can occur

28. Concurrency Control with Locking Methods • Lock • Guarantees exclusive use of a data item to a current transaction • T2 does not have access to a data item that is currently being used by T1 • Transaction acquires a lock prior to data access; the lock is released when the transaction is complete • Required to prevent another transaction from reading inconsistent data • Lock manager • Responsible for assigning and policing the locks used by the transactions

29. Lock Granularity • Indicates the level of lock use • Locking can take place at the following levels: • Database-level lock • Entire database is locked • Table-level lock • Entire table is locked • Page-level lock • Entire diskpage is locked • Row-level lock • Allows concurrent transactions to access different rows of the same table, even if the rows are located on the same page • Field-level lock • Allows concurrent transactions to access the same row, as long as they require the use of different fields (attributes) within that row

30. A Database-Level Locking Sequence • Good for batch processing but unsuitable for online multi-user DBMSs • T1 and T2 can not access the same database concurrently even if they use different tables

31. Table-Level Lock • T1 and T2 can access the same database concurrently as long as they use different tables • Can cause bottlenecks when many transactions are trying to access the same table (even if the transactions want to access different parts of the table and would not interfere with each other) • Not suitable for multi-user DBMSs

32. Page-Level Lock • An entire disk page is locked (a table can span several pages and each page can contain several rows of one or more tables) • Most frequently used multi-user DBMS locking method

33. Row-Level Lock • Concurrent transactions can access different rows of the same table even if the rows are located on the same page • Improves data availability but with high overhead (each row has a lock that must be read and written to)

34. Field-Level Lock • Allows concurrent transactions to access the same row as long as they require the use of different fields with that row • Most flexible lock buy requires an extremely high level of overhead

35. Binary Locks • Has only two states: locked (1) or unlocked (0) • Eliminates “Lost Update” problem – the lock is not released until the write statement is completed • Can not use PROD_QOH until it has been properly updated • Considered too restrictive to yield optimal concurrency conditions as it locks even for two READs when no update is being done

36. Shared/Exclusive Locks • Exclusive lock • Access is specifically reserved for the transaction that locked the object • Must be used when the potential for conflict exists – when a transaction wants to update a data item and no locks are currently held on that data item by another transaction • Granted if and only if no other locks are held on the data item • Shared lock • Concurrent transactions are granted Read access on the basis of a common lock • Issued when a transaction wants to read data and no exclusive lock is held on that data item • Multiple transactions can each have a shared lock on the same data item if they are all just reading it • Mutual Exclusive Rule • Only one transaction at a time can own an exclusive lock in the same object

37. Shared/Exclusive Locks • Increased overhead • The type of lock held must be known before a lock can be granted • Three lock operations exist: • READ_LOCK to check the type of lock • WRITE_LOCK to issue the lock • UNLOCK to release the lock • A lock can be upgraded from share to exclusive and downgraded from exclusive to share • Two possible major problems may occur • The resulting transaction schedule may not be serializable • The schedule may create deadlocks

38. Two-Phase Locking to Ensure Serializability • Defines how transactions acquire and relinquish locks • Guarantees serializability, but it does not prevent deadlocks • Growing phase, in which a transaction acquires all the required locks without unlocking any data • Shrinking phase, in which a transaction releases all locks and cannot obtain any new lock

39. Two-Phase Locking to Ensure Serializability • Governed by the following rules: • Two transactions cannot have conflicting locks • No unlock operation can precede a lock operation in the same transaction • No data are affected until all locks are obtained—that is, until the transaction is in its locked point

40. Two-Phase Locking Protocol

41. Deadlocks • Condition that occurs when two transactions wait for each other to unlock data • T1 needs data items X and Y; T needs Y and X • Each gets the its first piece of data but then waits to get the second (which the other transaction is already holding) – deadly embrace • Possible only if one of the transactions wants to obtain an exclusive lock on a data item • No deadlock condition can exist among shared locks • Control through • Prevention • Detection • Avoidance

42. How a Deadlock Condition Is Created

43. Concurrency Control with Time Stamping Methods • Assigns a global unique time stamp to each transaction • All database operations within the same transaction must have the same time stamp • Produces an explicit order in which transactions are submitted to the DBMS • Uniqueness • Ensures that no equal time stamp values can exist • Monotonicity • Ensures that time stamp values always increase • Disadvantage • Each value stored in the database requires two additional time stamp fields – last read, last update

44. Wait/Die & Wound/Wait Schemes • Wait/die • Older transaction waits and the younger is rolled back and rescheduled • Wound/wait • Older transaction rolls back the younger transaction and reschedules it • In the situation where a transaction is requests multiple locks, each lock request has an associated time-out value. If the lock is not granted before the time-out expires, the transaction is rolled back

45. Wait/Die and Wound/WaitConcurrency Control Schemes

46. Concurrency Control with Optimistic Methods • Optimistic approach • Based on the assumption that the majority of database operations do not conflict • Does not require locking or time stamping techniques • Transaction is executed without restrictions until it is committed • Acceptable for mostly read or query database systems that require very few update transactions • Phases are read, validation, and write

47. Concurrency Control with Optimistic Methods • Phases are read, validation, and write • Read phase – transaction reads the database, executes the needed computations and makes the updates to a private copy of the database values. • All update operations of the transaction are recorded in a temporary update file which is not accessed by the remaining transactions • Validation phase – transaction is validated to ensure that the changes made will not affect the integrity and consistency of the database • If the validation test is positive, the transaction goes to the writing phase. If negative, the transaction is restarted and the changes discarded • Writing phase– the changes are permanently applied to the database

48. Database Recovery Management • Database recovery • Restores database from a given state, usually inconsistent, to a previously consistent state • Based on the atomic transaction property • All portions of the transaction must be treated as a single logical unit of work, in which all operations must be applied and completed to produce a consistent database • If transaction operation cannot be completed, transaction must be aborted, and any changes to the database must be rolled back (undone)

49. Transaction Recovery • The database recovery process involves bringing the database to a consistent state after failure. • Transaction recovery procedures generally make use of deferred-write and write-through techniques

50. Transaction Recovery • Deferred write • Transaction operations do not immediately update the physical database • Only the transaction log is updated • Database is physically updated only after the transaction reaches its commit point using the transaction log information • If the transaction aborts before it reaches its commit point, no ROLLBACK is needed because the DB was never updated • A transaction that performed a COMMIT after the last checkpoint is redone using the “after” values of the transaction log